Ab-initio calculations are carried out to analyze the improvement of hydrogen storage properties, in Mg-based hydrides, through a double substitution (Mg(14)TMAIH(32) and Mg14TMLiH32 TM = Ti, Sc, Zn). The calculations are performed using the all-electron full-potential local-orbital minimum-basis scheme (FPLO9.00-34). Our results indicate a decrease of the heat of formation and desorption temperature, while preserving the gravimetric storage capacity around 7.6 wt%, these outcomes are probably due to the second substitution, Al or Li, which are characterized by their light weights. Discussions of the charge exchange and the density of states are done, to offer more possible explanations of our results. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.